Holographic storage systems are storage systems that use holographic storage media to store data. Holographic storage media includes photorefractive materials that can take advantage of the photorefractive effect described by David M. Pepper et al., in “The Photorefractive Effect,” Scientific American, October 1990 pages 62-74.
The index of refraction in photorefractive materials can be changed by light that passes through them. Holographic storage media also include photopolymers, such as those described in Coufal et al., “Photopolymers for Digital Holographic Storage” in Holographic Data Storage, 199-207 (2000), and photochromatic materials. By controllably changing the index of refraction in such materials, high-density, high-capacity, and high-speed storage of information in holographic storage media can be accomplished.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a data beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a data beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image.
Information can be encoded within the data beam in a variety of ways. One way of encoding information into a data beam is by using an electronic mask, called a spatial-light modulator (SLM). Typically, a SLM is a two dimensional matrix of pixels. Each pixel in the matrix can be directed to transmit or reflect light, corresponding to a binary 1, or to block light, corresponding to a binary 0. The data beam, once encoded by the SLM, is relayed onto the storage medium, where it intersects with a reference beam to form an interference pattern. The interference pattern records the information encoded in the data beam to the holographic storage medium.
The information recorded in the holographic storage medium is read by illuminating the storage medium with a reference beam. The resulting data beam is then typically imaged onto a sensor, such as a Charge Coupled Device (CCD) array or a CMOS active pixel sensor. The sensor is attached to a decoder, which is capable of decoding the data.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (“photopolymers”) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. A monomer polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
FIG. 1 illustrates the basic components of a holographic system 100. System 100 contains a SLM 112, a holographic storage medium 114, and a sensor 116. SLM 112 encodes beam 120 with an object image. The image is stored by interfering the encoded data beam 120 with a reference beam 122 at a location on or within holographic storage medium 114. The interference creates an interference pattern (or hologram) that is captured within medium 114 as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 122, depending on the particular reference beam employed. Data beam 120 typically passes through lenses 130 before being intersected with reference beam 122 in the medium 114. It is possible for reference beam 122 to pass through lenses 132 before this intersection. Once data is stored in medium 114, it is possible to retrieve the data by intersecting a reference beam 122 with medium 114 at the same location and at the same angle, wavelength, or phase at which a reference beam 122 was directed during storage of the data. The reconstructed data beam passes through one or more lenses 134 and is detected by sensor 116. Sensor 116, is for example, a charged coupled device or an active pixel sensor. Sensor 116 typically is attached to a unit that decodes the data.
Varying the angle of the reference beam during recording to store multiple holographic images in the same volume is called angle multiplexing. Each image is recorded in the same volume using a different reference beam angle. A large number of images can be stored in the same volume using angle multiplexing by varying the angle of the reference beam over a wide range.
However, varying the reference beam angle can increase the area of the holographic storage medium exposed by the reference beam. The area exposed by a reference beam that strikes the surface of the holographic storage medium depends upon the reference beam's angle of incidence with the storage medium (“the obliquity”). This area is related to the capacity of the holographic storage medium since the larger area exposed by the reference beam, the smaller the capacity of the holographic storage medium per unit volume. Accordingly, a need exists for a optical system that is capable of maintaining the size of the area exposed by a reference beam as the obliquity of the reference beam changes.
In the past, obliquity has been corrected using a complex set of prisms. The use of these prisms is discussed in Coufal et al., “Tamarack Optical Head Holographic Sorage” in Holographic Data Storage, 343-357 (2000). FIG. 2 shows an obliquity correction system using two prisms 226 and 228 and three lens component 230, 232 and 234. In FIG. 2, light beams 224 are reflected off of scanning mirror 222 onto first prism 226. The light beams exiting first prism 226 then proceed to second prism 228. The light beams exiting second prism 228 then proceed through lens components 230, 232 and 234.
The use of the complex prisms shown in FIG. 2 have the drawback of being difficult to manufacture and align. Accordingly, a need exists for an obliquity correction system that does not require the use of complex prisms.